One important application of filters is for filtration of drinking water. Source water typically has a pH of about 6 to 7, which varies depending upon environmental and other factors. Particles in the source water can be positively or negatively charged with various magnitudes of charge. Particles present in most naturally occurring source water are generally negatively charged.
Current regulations by the Environmental Protection Agency require source water to have a certain turbidity or clarity before it is suitable for drinking. These regulations also require the removal or deactivation of viruses and protozoan cysts from the water. Examples of cysts that must be treated are giardia and cryptosporidium. When ingested by humans these cysts can cause serious illness or death. Cryptosporidium cysts range from about 3 to about 5 microns in size and giardia cysts range from about 7 to about 12 microns in size, which makes them difficult to remove efficiently and economically with current filtration systems.
Conventional filters remove cysts and other particles of a small size using chemical coagulants. The chemical coagulants increase the size of the particles to a point at which they can be removed. During coagulation, small particles are agglomerated into larger particles by adding the chemical coagulants to the feed solution. Once agglomerates of a desired size are produced, the solution may be passed through a filter to filter out the agglomerates. Examples of water filtration media are sand, garnet and anthracite.
Chemical coagulation has several disadvantages. The mechanism for filtering the water is by physically straining particles from the feed solution which are larger than can pass through interstices between grains of the media. The media can only remove particles that are larger than the interstices. For example, sand filters can only remove particles greater than about 20 microns in size. Eventually, the particles held by the media seal off the interstices, reducing filtration efficiency.
Chemical coagulation is disadvantageous in that filtration occurs primarily at the surface of the bed, rather than throughout the bulk of the media, which limits the capacity of the filter. Chemical coagulation is also disadvantageous in view of the cost of the chemicals, the need to regulate the amount of chemicals despite a continuously changing feed stream and in view of a low flow rate. Disposing of chemical sludge waste is another concern.